This invention relates generally to a method and apparatus for cooling articles, and more specifically to substantially vaporizing a liquid a cryogen in a feed chamber and then circulating the vaporized cryogen through either a separate cooling chamber or through cooling circuits in a tool, such as a calibrator. The invention is particularly useful as an extrusion chiller, and also for chilling foods. Additionally, many other applications of the invention will become apparent to those skilled in the art upon a review of the following specification and drawings.
Historically, water has been utilized as the primary medium for cooling articles, including extrusions. For example, conventional extrusion chilling systems employ a xe2x80x9ccoolingxe2x80x9d chamber downstream from the extruder. The extrusion is fed through the cooling chamber, wherein the extrusion can be sprayed with water, or partially/fully submerged in water in order to chill the extrusion. Various other components may also be included in such systems, such as a vacuum sizing chamber intermediate the extruder and the cooling chamber. The vacuum sizing chamber can be used for hollow extrusions and employs a vacuum to assist the extrusion in maintaining its shape while it cools. Water can also be used in the vacuum chamber to cool the extrusion while the vacuum supports the shape.
Coolant mediums other than water which have been used in cooling processes can be referred to collectively as refrigerants, including cryogens. Cryogens include liquid nitrogen, liquid carbon dioxide, liquid air and other refrigerants having normal boiling points substantially below minus 50xc2x0 F. (xe2x88x9246xc2x0 C.). Prior art methods of cooling articles using cryogens disclose the benefits of fully vaporizing the cryogen into a gaseous refrigerant prior to contact with the articles to be cooled. Cryogens due to their extremely low boiling point, naturally and virtually instantaneously expand into gaseous form when dispersed into the air. This results in a radical consumption of heat. The resilient temperature can be reduced to hundreds of degrees below zero (Fahrenheit) in a relatively short time. However, prior methods of cooling fail to realize the advantages, both in increased efficiency and in improved system control, that can be achieved according to the invention by utilizing forced air convection in combination with the nitrogen or any other refrigerant. Some disadvantages of prior art cryogenic cooling systems include lower efficiency and limited options for controlling the cooling process. Such systems generally rely exclusively on the cooling effect of the refrigerant, to lower the ambient temperature and chill the article. Although prior art methods utilize forced convection to ensure complete vaporization of the cryogen, no methods use forced air convection to control the rate of cooling of the article by controlling the wind chill temperature. Consequently, the only control variable in the prior art methods to adjust (lower) the temperature is the introduction of liquid cryogen into the system. In contrast, the utilization of forced air convection adds a wide range of variable control to adjust the effective temperature, up or down, by controlling the velocity at which the refrigerant, is circulated over/around the article to be cooled.
The basis of forced air convection is the principle that blowing a refrigerant over a heated surface greatly enhances the transfer of heat from that surface. In the context of cold temperatures, this principle is probably better known indirectly, from the commonly used phrase xe2x80x9cwind chillxe2x80x9d temperature, which is frequently reported on TV or radio by weather announcers. In that context, wind chill temperature is what the temperature outside xe2x80x9cfeelsxe2x80x9d like, taking into account the ambient temperature and the prevailing velocity of the wind. The stronger (higher velocity) the wind, the lower the temperature xe2x80x9cfeels,xe2x80x9d compared to if there were no wind present. This principle is advantageously utilized according to the invention to significantly enhance the cooling efficiency of the system by creating, and controlling, xe2x80x9cwind chillxe2x80x9d temperature during the cooling process. As a result, the efficiency of the process can be increased while simultaneously reducing the size, which is typically the length, of the cooling system.
Another type of prior art cooling system utilizes a device called a xe2x80x9ccalibrator,xe2x80x9d and typically multiple such calibrators, to cool extrusions. A calibrator is a tool which generally has a central opening through which the extrusion is fed, the central opening having a surface which is generally in contact with the surface of the extrusion as it is fed therethrough. As a result of contact with the surface of the extrusion, heat is conducted to the calibrator away from the extrusion thus cooling the extrusion. To enhance the heat transfer from the extrusion, cooling circuits, internal passages, are provided in the calibrator through which a coolant is circulated. Typically, the coolant is water, but liquid nitrogen is also known to have been used to some degree. However, circulating liquid nitrogen through the cooling circuits has met with some difficulties regarding contact of the liquid nitrogen with the calibrators. Moreover, it has been found that it is preferable to first vaporize a liquid cryogen, such as liquid nitrogen, and then to circulate the super cold vapor/refrigerant through the cooling circuits instead of the liquid cryogen, which thus requires a system for vaporizing the liquid cryogen prior to circulation through the cooling circuits of the calibrator.
Accordingly, there is a need for a method and apparatus for cooling articles which can provide improved efficiency and can also reduce the size of the cooling system, and also an alternative cooling system for circulating vaporized cryogen through cooling circuits in calibrators.
A method and apparatus for cooling articles is provided which can utilize the dispersion of a liquid cryogen into a feed chamber wherein the liquid cryogen is substantially vaporized and then circulated through a cooling chamber containing the article to be cooled. The vaporized cryogen can be further circulated though the cooling chamber at a controllable velocity, over/around the surface of the article to be cooled, in order to regulate the rate of cooling the article by controlling the wind chill temperature, based upon the principles of forced air convection.
A presently preferred cryogen is liquid nitrogen. The liquid nitrogen can be dispersed into a feed chamber in a controlled manner using an valve, which itself can be operated by a controller, such as a microprocessor. Since the temperature in the feed chamber is much higher than the boiling point of the liquid nitrogen, a high BTU (British Thermal Unit) and expansion rate is captured thereby producing an extremely effective refrigerant. The feed chamber can be communicated with a cooling chamber into which the vaporized cryogen can be circulated by a fan, or other device for circulating air and/or vaporized cryogen. Either the feed chamber or the cooling chamber can be vented to dissipate pressure generated as the liquid nitrogen rapidly expands to gaseous form. The fan can preferably be a variable speed fan, or other variable speed circulation device, for circulating the vaporized cryogen through the system at a controllable velocity to take advantage of principles of forced air convection. The fan can be located in the feed chamber to aid in substantially vaporizing the liquid cryogen. However, considering the relatively high temperature utilized in the cooling chamber compared to the boiling point of the cryogen, even without the fan, the liquid cryogen will virtually completely and instantaneously vaporize as it is injected into the feed chamber. The fan can be operated by the controller which can regulate the speed of the fan to provide improved temperature control over the system by controlling the wind chill temperature in the cooling chamber. The system can also include a temperature sensor, connected to the controller, for monitoring the temperature in the cooling chamber, and to calculate the wind chill temperature. A heating device can be provided to increase the temperature in the cooling chamber, if needed. The speed of the fan can be controlled by the microprocessor to circulate the refrigerant at a high volume (CFM) to maximize the cooling efficiency, thereby minimizing cryogen consumption. Essentially, the rate of cooling of the article can be increased for a given amount of cryogen dispersed into the feed chamber by increasing the speed of the fan. Another way to express this concept is to say that the xe2x80x9ceffective temperaturexe2x80x9d in the chamber can be reduced by increasing the speed of the fan. The articles to be cooled can be delivered into the cooling chamber by means of a conveyor belt, or various other ways of feeding articles, for example pulling extrusions, through the cooling chambers.
The cooling system can also employ a plurality of cooling chambers, preferably adjacent, each of which can be individually controlled by one or more controllers. The controllers can manage the speed of the fan and the nitrogen injection for each individual cooling chamber, thereby providing for maximum heat exchange rates for efficiency and effectiveness. Each cooling chamber can be equipped with its own temperature sensor, nitrogen injection valve to control the introduction of nitrogen into the cooling chamber, and variable speed fan for circulating refrigerant through the cooling chamber.
In general operation, the temperature sensor detects the temperature in the cooling chamber, or of the circulated refrigerant, and feeds the information to the controller. The controller can be programmed with a desired temperature to which the temperature inside the cooling chamber is to be regulated. The controller can also control the nitrogen injection valve and the speed of the fan to cause the temperature in the cooling chamber to correspond to the desired temperature. An equation for calculating the xe2x80x9ceffective temperature,xe2x80x9d i.e. wind chill temperature, from the speed of the fan and the ambient temperature in the cooling chamber can be programmed into the microprocessor. The speed of the fan can thus be regulated to increase or decrease the rate of cooling of the article, by adjusting the effective temperature in the cooling chamber, in order to maximize the efficiency of the cooling system. Principles of forced air convection can thus be utilized to increase cooling efficiency while minimizing the consumption of nitrogen. The fan additionally permits improved system control over the effective temperature in the cooling chamber.
In some embodiments, the cooling chamber may be provided with a diffuser positioned between the article to be cooled and the initial dispersion of refrigerant into the cooling chamber. The diffuser can have an xe2x80x9cinletxe2x80x9d into which flow from the fan is channeled, such as by a duct. As the nitrogen is dispersed into an area in the diffuser and expands into a gaseous refrigerant, the fan circulates the refrigerant through the diffuser. The diffuser can aid in more equally distributing the refrigerant around all sides of the article. The diffuser can have multiple openings directing the refrigerant towards the middle of the cooling chamber where the article to be cooled is located. Accordingly, the diffuser can provide a more even distribution of the refrigerant around all sides of the article for more efficient and evenly distributed cooling.
Another embodiment of the invention is a cooling system which, while not utilizing wind chill temperatures, is particularly adapted to vaporize a liquid cryogen and circulate the vapor/refrigerant through internal cooling circuits, commonly provided in metal tools for cooling either the tool itself or an article within the tool. A specific example of such a tool is a calibrator, which is commonly used to cool extruded articles. An additional embodiment of the invention can be a combination of a calibrator based cooling system and a cooling chamber based system wherein the calibrators are enclosed within the cooling chamber, or chambers. In the combined system, the calibrators, through which an extrusion is passed to be cooled, is itself, along with the extrusion, cooled within a cooling chamber.
Other details, objects, and advantages of the invention will become apparent from the following detailed description and the accompanying drawings figures of certain embodiments thereof.